Why haven't I got my Period?

I haven't got my period in 2 and a half months, however I have done 3 pregnancy tests at my dr. and they are coming back negative.... I have a lot of symptoms of being pregnant (There the same as when I was pregnant the first time.) 2 months ago I came off of Yasmin birth control and last month I was put on Portia birth control...because of this do you think I could be pregnant and my hormones be all off wack, so the tests are ccomingack negative or could this be something else?